1. Field of the Invention
The present invention relates to the field of projected images, particularly overhead projected images and overhead projected images formed from inks or toners applied to transparency receptor films by electrostatic imaging processes, such as electrophotography.
2. Background of the Art
Electrophotography forms the technical basis for various well-known imaging processes, including photocopying and some forms of laser printing. Other imaging processes use electrostatic or ionographic printing. Electrostatic printing is printing where a dielectric receptor or substrate is “written” upon imagewise by a charged stylus, leaving a latent electrostatic image on the surface of the dielectric recpetor. This dielectric receptor is not photosensitive and is generally not re-useable. Once the image pattern has been “written” onto the dielectric receptor in the form of an electrostatic charge pattern of positive or negative polarity, oppositely charged toner particles are applied to the dielectric receptor in order to develop the latent image. An exemplary electrostatic imaging process is described in U.S. Pat. No. 5,176,974.
In contrast, electrophotographic imaging processes typically involve the use of a reusable, light sensitive, temporary image receptor, known as a photoreceptor, in the process of producing an electrophotographic image on a final, permanent image receptor. A representative electrophotographic process involves a series of steps to produce an image on a receptor, including charging, exposure, development, transfer, fusing, and cleaning, and erasure.
In the charging step, a photoreceptor is covered with charge of a desired polarity, either negative or positive, typically with a corona or charging roller. In the exposure step, an optical system, typically a laser scanner or diode array, forms a latent image by selectively exposing the photoreceptor to electromagnetic radiation, thereby discharging the charged surface of the photoreceptor in an imagewise manner corresponding to the desired image to be formed on the final image receptor. The electromagnetic radiation, which may also be referred to as “light”, may include infrared radiation, visible light, and ultraviolet radiation, for example.
In the development step, toner particles of the appropriate polarity are generally brought into contact with the latent image on the photoreceptor, typically using a developer electrically-biased to a potential opposite in polarity to the toner polarity. The toner particles migrate to the photoreceptor and selectively adhere to the latent image via electrostatic forces, forming a toned image on the photoreceptor.
In the transfer step, the toned image is transferred from the photoreceptor to the desired final image receptor; an intermediate transfer element is sometimes used to effect transfer of the toned image from the photoreceptor with subsequent transfer of the toned image to a final image receptor. The transfer of an image typically occurs by one of the following two methods: elastomeric assist (also referred to herein as “adhesive transfer”) or electrostatic assist (also referred to herein as “electrostatic transfer”).
Elastomeric assist or adhesive transfer refers generally to a process in which the transfer of an image is primarily caused by balancing the relative energies between the ink, a photoreceptor surface and a temporary carrier surface or medium for the toner. The effectiveness of such elastomeric assist or adhesive transfer is controlled by several variables including surface energy, temperature, pressure, and toner rheology. An exemplary elastomeric assist/adhesive image transfer process is described in U.S. Pat. No. 5,916,718.
Electrostatic assist or electrostatic transfer refers generally to a process in which transfer of an image is primarily affected by electrostatic charges or charge differential phenomena between the receptor surface and the temporary carrier surface or medium for the toner. Electrostatic transfer may be influenced by surface energy, temperature, and pressure, but the primary driving forces causing the toner image to be transferred to the final substrate are electrostatic forces. An exemplary electrostatic transfer process is described in U.S. Pat. No. 4,420,244.
In the fusing step, the toned image on the final image receptor is heated to soften or melt the toner particles, thereby fusing the toned image to the final receptor. An alternative fusing method involves fixing the toner to the final receptor under high pressure with or without heat. In the cleaning step, residual toner remaining on the photoreceptor is removed.
Finally, in the erasing step, the photoreceptor charge is reduced to a substantially uniformly low value by exposure to light of a particular wavelength band, thereby removing remnants of the original latent image and preparing the photoreceptor for the next imaging cycle.
Two types of toner are in widespread, commercial use: liquid toner and dry toner. The term “dry” does not mean that the dry toner is totally free of any liquid constituents, but connotes that the toner particles do not contain any significant amount of solvent, e.g., typically less than 10 weight percent solvent (generally, dry toner is as dry as is reasonably practical in terms of solvent content), and are capable of carrying a triboelectric charge.
A typical liquid toner composition generally includes toner particles suspended or dispersed in a liquid carrier. The liquid carrier is typically nonconductive dispersant, to avoid discharging the latent electrostatic image. Liquid toner particles are generally solvated to some degree in the liquid carrier (or carrier liquid), typically in more than 50 weight percent of a low polarity, low dielectric constant, substantially nonaqueous carrier solvent. Liquid toner particles are generally chemically charged using polar groups that dissociate in the carrier solvent, but do not carry a triboelectric charge while solvated and/or dispersed in the liquid carrier. Liquid toner particles are also typically smaller than dry toner particles. Because of their small particle size, ranging from about 5 microns to sub-micron, liquid toners are capable of producing very high-resolution toned images. This distinguishes dry toner particles from liquid toner particles.
A typical toner particle for a liquid toner composition generally comprises a visual enhancement additive (for example, a colored pigment particle) and a polymeric binder. The polymeric binder fulfills functions both during and after the electrophotographic process. With respect to processability, the character of the binder impacts charging and charge stability, flow, and fusing characteristics of the toner particles. These characteristics are important to achieve good performance during development, transfer, and fusing. After an image is formed on the final receptor, the nature of the binder (e.g. glass transition temperature, melt viscosity, molecular weight) and the fusing conditions (e.g. temperature, pressure and fuser configuration) impact durability (e.g. blocking and erasure resistance), adhesion to the receptor, gloss, and the like.
Polymeric binder materials suitable for use in liquid toner particles typically exhibit glass transition temperatures of about −24° C. to 55° C., which is lower than the range of glass transition temperatures (50–100° C.) typical for polymeric binders used in dry toner particles. In particular, some liquid toners are known to incorporate polymeric binders exhibiting glass transition temperatures (Tg below room temperature (25° C.) in order to rapidly self fix, e.g., by film formation, in the liquid electrophotographic imaging process; see e.g. U.S. Pat. No. 6,255,363. However, such liquid toners are also known to exhibit inferior image durability resulting from the low Tg (e.g. poor blocking and erasure resistance) after fusing the toned image to a final image receptor.
In other printing processes using liquid toners, self-fixing is not required. In such a system, the image developed on the photoconductive surface is transferred to an intermediate transfer belt (“ITB”) or intermediate transfer member (“ITM”) or directly to a print medium without film formation at this stage. See, for example, U.S. Pat. No. 5,410,392 to Landa, issued on Apr. 25, 1995; and U.S. Pat. No. 5,115,277 to Camis, issued on May 19, 1992. In such a system, this transfer of discrete toner particles in image form is carried out using a combination of mechanical forces, electrostatic forces, and thermal energy. In the system particularly described in the '277 patent, DC bias voltage is connected to an inner sleeve member to develop electrostatic forces at the surface of the print medium for assisting in the efficient transfer of color images.
The toner particles used in such a system have been previously prepared using conventional polymeric binder materials, and not polymers made using an organosol process. Thus, for example the '392 patent states that the liquid developer to be used in the disclosed system is described in U.S. Pat. No. 4,794,651 to Landa, issued on Dec. 27, 1988. This patent discloses liquid toners made by heating a preformed high Tg polymer resin in a carrier liquid to an elevated temperature sufficiently high for the carrier liquid to soften or plasticize the resin, adding a pigment, and exposing the resulting high temperature dispersion to a high energy mixing or milling process.
Although such non self-fixing liquid toners using higher Tg (Tg generally greater than or equal to about 60° C.) polymeric binders should have good image durability, such toners are known to exhibit other problems related to the choice of polymeric binder, including image defects due to the inability of the liquid toner to rapidly self fix in the imaging process, poor charging and charge stability, poor stability with respect to agglomeration or aggregation in storage, poor sedimentation stability in storage, and the requirement that high fusing temperatures of about 200–250° C. be used in order to soften or melt the toner particles and thereby adequately fuse the toner to the final image receptor.
To overcome the durability deficiencies, polymeric materials selected for use in both nonfilm-forming liquid toners and dry toners more typically exhibit a range of Tg of at least about 55–65° C. in order to obtain good blocking resistance after fusing, yet typically require high fusing temperatures of about 200–250° C. in order to soften or melt the toner particles and thereby adequately fuse the toner to the final image receptor. High fusing temperatures are a disadvantage for dry toners because of the long warm-up time and higher energy consumption associated with high temperature fusing and because of the risk of fire associated with fusing toner to paper at temperatures approaching the autoignition temperature of paper (233° C.).
In addition, some liquid and dry toners using high Tg polymeric binders are known to exhibit undesirable partial transfer (offset) of the toned image from the final image receptor to the fuser surface at temperatures above or below the optimal fusing temperature, requiring the use of low surface energy materials in the fuser surface or the application of fuser oils to prevent offset. Alternatively, various lubricants or waxes have been physically blended into the dry toner particles during fabrication to act as release or slip agents; however, because these waxes are not chemically bonded to the polymeric binder, they may adversely affect triboelectric charging of the toner particle or may migrate from the toner particle and contaminate the photoreceptor, an intermediate transfer element, the fuser element, or other surfaces critical to the electrophotographic process.
Since the introduction of electrophotographic copying and printing machines using dry toner powder particles to develop electrostatic images, there has been a continuing emphasis on toner image transfer with faithful, high quality, durable fused image reproduction on the surface of a receptor sheet. Previous studies related to the quality of images produced by transfer of toner powder, from imaging drums of electrophotographic copiers and printers to suitable recording sheets, focused attention on the bond formed between the powder and the recording sheet. Having demonstrated sufficient adhesion, measurement of optical density indicated the intensity of the image formed on the recording sheet, as shown by U.S. Pat. Nos. 5,451,458. 5,302,439 discloses a recording sheet which comprises a substrate and a coating thereon Materials from the various groups increase the adhesion of toner powder to the recording sheet.
More recently, attention has turned to improvement of the projected transparency of images formed by fusing dry toner particles to transparent receptor films, Poor transparency of fused dry toner images is believed to result from multiple light scattering from solid particles (e.g. pigment particles) having a volume mean diameter generally larger than approximately 0.5–1 micron, such that light projected through the fused toner layer undergoes multiple scattering from the solid particles. Alternatively, multiple light scattering may occur at the surfaces and edges of the fused transparency image when the projector's light source is transmitted through the fused toned image on the transparency receptor.
The problem of light scattering is particularly undesirable and tends to be more noticeable with the use of colored pigments. In the use of black and white images, light scatter tends to only cause increased opacity or increased transmission optical density of the projected image, which is minimally problematic, as the projected image actually appears more black, However, when light is scattered by multi-colored pigment particles, there is a color shift effected, and this results in not only altering the transmitted optical density and projected imagte opacity, but also the color content itself. The imaging process must retain the ability to provide high fidelity to the intended colors and high color quality when the fused toned image on the receptor film is projected, or the image quality will be seriously degraded. For example, one of the most apparent scattering effects is noted in overhead projection of toned yellow colors, where the projected yellow image is often “muddy,” appearing as gold, light green, brown or even black, depending upon smoothness of the fused toner film and the extent of light scattering from the surface of the fused toned image.
Various approaches have been used in the art to improve the projected transparency of fused dry toner images. One approach involves alteration of the toner composition to improve uniformity of the fused toner layer on the transparent image receptor. U.S. Pat. No. 5,635,325 discloses a core/shell toner for developing electrostatic images including a binder resin, a colorant and an ester wax, wherein the core melts and acts as a release agent during fusing, eliminating the need for silicone based release agents to be applied to the fuser rolls.
Other approaches to improve the projection quality of fused dry toner images on transparency receptors involve use of special coatings on the transparency receptor to improve coalescence of the toner powders into a smooth, uniform layer on the receptor. U.S. Pat. Nos. 5,208,093, 4,298,309 and 5,635,325 disclose a variety of solutions to achieve miscibility of the coated film with the toner while maintaining low melt viscosity. U.S. Pat. No. 5,451,458 discloses a recording sheet which comprises a substrate and a coating thereon containing a binder selected from polyesters, polyvinyl acetals, vinyl alcohol-vinyl acetal copolymers, polycarbonates, and mixtures thereof, and an additive having a melting point of less than about 65 degree C. and a boiling point of more than about 150 degree C. U.S. Pat. Nos. 6,391,954 and 6,296,931 describe a recording sheet including an additive, referred to as a compatibilizer, to improve the quality of images formed by toner powder development of electrostatic charge patterns.
Still other approaches are directed at improved toner transparency using special fixing methods for fusing the dry toner powder to the receptor sheet. U.S. Pat. No. 5,824,442 describes the use of special toners in such a fixing method. U.S. Pat. No. 5,519,479 describes a fusing or fixing device for use in an electrophotographic apparatus, comprising a pair of pressing means opposing each other to form therebetween a nip through which an image supporting member supporting an unfixed toner image is passed so that the toner image is fixed to the image supporting member, wherein one of the pressing means which contacts the toner image on the image supporting member has a layer formed of a soft matrix and granular particles dispersed in the matrix and having greater hardness than the matrix, whereby fine irregularities are formed on toner surfaces on the image supporting member by the granular particles under application of pressure during fixing. This is clearly contraindicated as a solution against light scattering that shifts color balance and fidelity.
For the case of fused liquid toner images on transparent receptor sheets, all of the previously described problems may occur. Poor fused toner adhesion, unacceptable projected image transparency due to light scattering by oversized pigment particles or surface irregularities in the fused toner layer, and poor color fidelity in multi-color fused toner images remain problematic. In addition, the difficulty of producing durable, transparent, multi-colored fused liquid toner images on transparency receptors is compounded by the fact that a substantial amount of carrier liquid is typically present in the toned image prior to fusing on the final transparency receptor. The latter issue is particularly problematic for transparencies produced using liquid electrophotographic imaging processes that make use of an electrostatic transfer assist to transfer the toned image to the final image receptor, because a substantial amount of carrier liquid is required in the toned image in order to effect electrostatic transfer. This carrier liquid may have adverse effects on the durability of the fused liquid toner image if it remains in the fused image. Alternatively, removal of the carrier liquid may have adverse effects on the transparency and color fidelity of the projected liquid toned image.
The art continually searches for way of improving the durability, projected transparency and projected color fidelity of liquid toned images fused on transparent receptors. The art also searches for improved methods of producing multi-colored, fused liquid toned images on transparency receptors using electrophotographic imaging processes having an electrostatic image transfer assist for transferring the toned image to the transparency receptor.